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# Monthly Archives: May 2012

## Size of a soap bubble when charged

What will happen to the size of the soap bubble if the bubble is given charges ?

Answer: When a soap bubble is charged, its size increases.

Under normal condictions, a soap bubble is in equilibrium un der two opposing forces – the force of surface tension which tries to compress it and the force due to excess pressure which tries to expand it.

When charged, the force of repulsion among the like charges will try to expand the soap bubble further resulting in an increase in size of the bubble.

## WORKING OF A TRANSFORMER

Sir,im not understanding how transformer works?while current is going from one station to other we keep high voltage and very less current to decrease power loss but current is proportional to voltage how can u make low current using high voltage?

A transformer works on the principle of mutual induction. [mutual induction is the phenomenon of production of emf in a coil due to the change in the strength of current through a neighbouring coil]

Transformers are of two kinds – step up and step down.

A step up transformer converts low voltage (high current) ac to high voltage (low current) ac.

In the case of an ideal transformer, there is no power loss.

In that case,

input power = out power

Vin Iin = Vout Iout

[This equation shows how an increase in output voltage results in a decrease in out put current]

Long distance transmission of ac requires low  current as the heat dissipated during transmission is proportional to the square of current.

If we use a step-up transformer , it steps up the voltage and consequently steps down the current so that VI is the same.

In the case of real transformers, there are some power losses within the transformner, however, an increase in output current through mutual induction will essentially result in a decrease in output current.

Hope that the matter is clear now. In case of doubt, please post them as comments to this post.

## CBSE Class 12 Results will be announced on 28 May 2012

CBSE has announced that the Results of CBSE Class 12 results (AISSCE 2012) will be published on 28 May 2012 at 11 am.

Wish you all good luck.

Visit http://cbseresults.nic.in and http://results.gov.in/result2announcedaca.aspx for updated information.

## Teachers on Vacation – The Answers may be delayed !

HELLO WORLD !

As the teachers associated with www.askphysics.com are on vacation, you may find a delay in getting your questions answered.

## Waves – Some numerical Problems and doubts

1a) two tunning forks when soundedtogether produce 3 beats per second on loading one of them with a little wax 20 beats are heard per second.find its frequency if that of other is 386 Hz.
1 b) calculate velocity of sound in a gas in which two wave length 2.04m produce 20 beat in 6 second.
1 c)a string 1m long with mass 0.1 gm/cm is under aa tension of 400 N find its fundamental frequency.
1d)a steel rod 100 cm long is clamped at middle of it.fundamental frequency of longitudinal vibration of the rod is given to be 2.53kHz.what is speed of sound in steel

## Calculating g on Moon

An astronaut on the moon wishes to calculate the local g in the area. He sets up a wire with a mass of 1.7 grams and hanging from it an object with mass 4.98 kg. He sends some kind of pulse down the wire and calculates that it takes 40.86 ms to traverse the wire. What is g? You can neglect the mass of the wire when calculating the tension.

That was everything that was given in the problem. I have not a clue how to solve it. What I did was just use the equation of T=2pi[sqrt(l/g)] to solve for g and hope that the wire acts like a spring or something.

I’m totally sure that’s incorrect, but hey I’m not an astrophysicist.

(Response from Members and visitors awaited)

Posted by Ashish Dharva

## Numerical problem from electrostatics

A small liquid drop is held steady in an electric field of intensity E,the charge of the liquid drop is q when the field is reversed what is the new accelaration,if viscous force is negligible?